In general, channel estimation is a vital technique used in receivers in wireless communication systems. The receiver can effectively recover transmitted information as long as it keeps track of the varying radio propagation channel conditions. The effect of the channel on the transmitted information is therefore estimated in order for the receiver to be able to correctly recover the transmitted information.
For example, next generation wireless systems such as 3GPP LTE (Rel. 8, 9) and 3GPP LTE-Advanced (Rel. 10) employ coherent demodulation techniques based on OFDM (Orthogonal Frequency Division Multiplexing) modulation. The demodulation of the received signal requires knowledge of the channel between the transmitter and the receiver, i.e. a mathematical model of the disturbance or distortion applied to the transmitted signal by the propagation environment.
In order to estimate the channel, a functional block usually called channel estimator is placed in the receiver. In order to allow channel estimation, a training-based approach is normally adopted in which the transmitter sends a reference signal, also sometimes referred to as a pilot, which is known to both the transmitter and receiver on a regular basis. The channel estimator then generates a model of the channel, based on the received distorted reference signal, to enable proper demodulation and decoding of the received signal.
Several algorithms have been proposed in literature for channel estimation. In the exemplary context of OFDM, a promising technique includes estimating the CIR (Channel Impulse Response) which corresponds to the representation of the channel in the time domain. The CIR may be transformed to the CTF (Channel Transfer Function) in the frequency domain by use of a DFT (Discrete Fourier Transform) [1-2]. The advantage of this technique is that the energy of the channel is usually concentrated in a limited number of samples. The operation of then removing the weakest samples is beneficial because it reduces the estimation noise without significantly distorting the CTF.
Another promising family of channel estimators is similar to the above described DFT based estimators, but is based on DCT (Discrete Cosine Transformation) instead of DFT [3-5]. The advantage of the DCT transformation is that it tends to focus the energy of the signal on even fewer samples, compared to DFT.
However, with the ever increasing demands for more efficient wireless communication there is a general need for further improvements of the existing channel estimation techniques.